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Abstract
In this paper, we proposed and demonstrated a new design of marine signal light based on a phosphor-converted white light emitting diode. The light pattern was shaped in the vertical and horizontal directions, respectively, by a collimating total internal reflection lens with cylindrical lens array and a slanted shoulder to fit the requirement in the regulation. The vertical intensity distribution was collimated to 9° through an appropriate volume of the total internal reflection lens to fit the etendue of the light source. The horizontal intensity distribution was shaped with a cylindrical lens array to enlarge the horizontal to the divergent angle, and a specially-designed slanted shoulder to reduce the total internal reflection loss and extend the divergent angle from 45° to 67° with a flat-top shaping light pattern. As a result, only five pieces of the light emitting diode module around 6W is enough to build up a marine signal light to reach a distance of 8 nautical miles.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Marine signal light is an important luminaire providing a guideline to sailors in handling a ship sailing on the sea. The request of a marine signal light is the accuracy in controlling the divergent angle in the horizontal and vertical directions. Owing to wide range of the sea, a marine signal light is expected to project the light into long distance, e.g., 8 nautical miles (NM) or longer. To achieve this property, the divergent angle along the vertical angle must be very limited, which could be no larger than 10°, while the light needs to cover 360° of a round view along the horizontal direction. Traditionally, the design of the marine signal light is to use a lens to collimate the light emitted by the light source and a cylindrical lens of the Fresnel-lens type to wide-spread the light in horizontal direction [1]. If the divergent angle of the light source is wide enough, a vertical cylindrical Fresnel lens is necessary to achieve the optical request while keeping the horizontal divergent angle as wide as possible. If the divergent angle of the light source does not fit the request in both vertical and horizontal directions, a complicated optics design is needed. Nowadays, the most light sources have been changed to solid-state light source, such as light emitting diode (LED). LED has been become the most important light source in general lighting as well as special lighting, owing to its advantages in high efficiency, vivid color, small size, robustness, low operation voltage, fast response and mechanical stability [2–7]. Phosphor-converted white LED (called pcW-LED) is based on blue LED die covering with down-conversion phosphor to perform white light [8–10]. The emission spectrum of the phosphor, phosphor concentration and the thickness of the phosphor layer can be adjusted to fit the color in cool white, warm white or in between. For the application to a marine signal light, pcW-LED as well as single-color LED is equipped with similar advantages. Therefore, in the design of the marine signal light, the color of luminaire can be adjustable by changing the LED at different colors.
In this paper, we will present a new design of marine signal light with high-power pcW-LED. The design will start from the request by the regulation, selection of a pcW-LED, the light source model, the optical design and the verification with experimental measurement.
2. Regulation
The design refers to the regulation for E-200-1 Marine Signal Lights by the International Association of Lighthouse Authorities (IALA) [11]. In the regulation, there are three main requests in the optical property, including chromaticity, divergent angle and illumination range. In the color requirement, the E-200-1 Marine Signal Lights request the color should fall in to the range marked in 1931 CIE chromaticity diagram [12], as shown in Fig. 1. The color range coordinates are listed in Table 1.
Fig. 1. The color range definition of the IALA E-200-1 Marine Signal Lights marked in 1931 CIE chromaticity diagram [12].
Table 1. The chromatic coordinates for the ranges in all acceptable colors.
In the requirement of the illumination range, the regulation of E-200-1 Marine Signal Lights define a factor of atmosphere transmission factor (T)
3. Light source and the optical model
In order to have small divergent angle and to keep the luminaire size in an acceptable size, the pcW-LED should have large flux density with relatively small chip size. Cree XP-G2 LED is a commercialized pcW-LED which meets the light source requirement, and the chip size is about 1.4 × 1.4 mm2 [13]. Because all the optical elements will be located at the mid-field range of the light source, we need to build up a precise optical mid-field model [14–18]. Mid field is the range between the starting point out of the near field in physics, and the far field. In building up the optical model, we need to have the detailed LED package structure, including chip size, phosphor area and the encapsulation lens profile. If the phosphor coverage is by conformal scheme, the phosphor coverage area can be regarded as the effective light emitting area. Usually, the light emission is similar to a Lambertian light source. Accordingly, we can build up the basic model of the pcW-LED. Through the angular intensity function measurement, and make a normalized cross-correlation (NCC) calculation [19], we can determine if the mid-field model is good or not. Generally, the criteria of the NCC is 99% in the cases of precise light pattern or extremely small divergent angle. Figure 2 shows the pcW-LED and its geometrical structure. Figure 3 shows the NCC's at different mid-field distances, where all the NCC's are higher than 99%, and it means that the light source model is precise enough for high-quality optical design.
Fig. 3. The measurement and the corresponding simulated intensity distribution at the mid-field range of (a) 15 mm, (b) 30 mm and (c) 50 mm.
4. Optical design
The first thinking is to achieve uniform intensity distribution along horizontal direction with a Lambertian light pattern, which is a typical light pattern of the most high-power pcW-LEDs. However, there will be some difficulty when the vertical divergent angle needs to be tightened to as small as 10°. If we use a cylindrical lens to squeeze the vertical light pattern to 10°, the Lambertian light pattern will be destroyed along the horizontal direction. Figure 4 shows a comparison of the intensity distribution along horizontal direction between a Lambertian light pattern and vertical-squeezed light pattern based on a Lambertian light source. Obviously, simply squeezing the vertical light pattern will also squeeze the horizontal divergent angle of FWHM from 120° to 40°, and it is not well to a round-view design. Then the optical design starts from collimation of the LED light and then to squeeze the vertical light pattern to fit the regulation. More effort is to use cylindrical lens to extend the horizontal light pattern to larger than a certain divergent angle and shape the light pattern to fit the regulation.
Fig. 4. Comparison of the horizontal intensity distribution for light patterns with and without vertical–squeezing lens based on a Lambertian light source.
Design of the collimating reflector is based on the precise optical model of the pcW-LED. The reflector is a TIR lens, where TIR means total internal reflection [20–23]. There are two structures in the TIR lens. First is a small positive lens along the optical axis, which is to collimate the lights around the optical axis. In this design, the small angle is between −20° and 20° of the LED light. The second is a composite parabolic reflector, which is used to collimate the emitting lights at the larger angle. In the TIR lens, the hollow cavity will displace the LED die at different heights of the reflector. Thus for ±21° to ±90° of the divergent angle of the LED light, we design multi parabolic reflective surfaces per angluar spacing of 10° and combine these parabolic reflective surfaces to form the composite parabolic reflector. The composite parabolic reflector is useful to tighten the divergent angle. The pcW-LED is located at the focus of the parabolic reflector and the small positive refraction surface. Because there is a hemisphere encapsulation lens on the phosphor, the effective light source is slightly below the real phosphor layer owing to a virtual imaging by the encapsulation lens. Theoretically, the encapsulation lens will not change etendue. [24] The lateral extension of the phosphor layer makes the effective emitting surface larger than a point source, so that the TIR lens cannot well collimate the emitting light. The limit of the vertical divergent angle depends on the volume of the TIR lens, so the etendue management of the light source and the collimating lens is important. In the design, the size of 35.67 mm (diameter) and 19 mm (height) of the TIR collimation lens is capable to collimate the vertical divergent angle to a range around 8° to 10° to meet the regulation. Then what we have to do in adjusting the divergent angle is to reduce the defocus of the light source by longitudinally shifting the pcW-LED to obtain a suitable divergent angle, as shown in Fig. 5.
Fig. 5. (a) The structure of the collimating TIR lens, (b) the design concept, and (c) the vertical intensity distribution.
To take care of the horizontal round-view illumination, we need to calculate the amount of the pcW-LED to satisfy the uniformity. If the number of the LED is large, it could increase cost. If the number of the LED is only 3 (or 4), each LED should take care of 120° (or 90°), and it will be difficult in optical design. In considering the design difficulty and the cost, we choose five pieces of the pcW-LED to confirm whether the amount of the pcW-LED is intense enough to support horizontal round-view illumination or not. Then each pcW-LED should illuminate a light cone spreading a range of 72°. To tighten the optical element, we design a slim cylindrical lens array on the exit face of the TIR lens to spread the light pattern and increase the horizontal divergent angle. The lens is so-called CLA-TIR lens. To optimize the lens structure, we adjust the width and height of the cylindrical lens [25–26], as shown in Fig. 6, where the width is fixed at 4 mm and the height is changed from 1 mm to 1.6 mm. The optimized vertical FWHM angle is 9°, and the horizontal FWHM angle varies from 41° to 45°. The horizontal light patterns shown in Fig. 6 is like a triangle shape, which is not good to form uniform round-view illumination. The light at larger divergent angle suffers from total internal reflection within the cylindrical lens, and serious backward reflection causes illumination efficiency drop to a low level of 63%. In such a design, the horizontal intensity distribution is not wide enough to cover the round-view design in the case of five pieces LED. We found that many lights were reflected backward on two lateral sides. To solve this problem, we replaced the cylindrical lens on two lateral sides by a pair of slanted shoulders (then so-called CLAS-TIR lens), which redirected the incident light propagated forward at larger angle and reduce energy loss by total internal reflection on the top surface, as shown in Fig. 7. The horizontal angle of FWHM was extended from 45° to 67° with a flat-top shaping light pattern, which could support high intensity uniformity along horizontal direction. Besides, the illumination efficiency increased to 84.2%.
Fig. 6. The simulated intensity distribution in the vertical (red line) and horizontal (back line) directions of the CLA-TIR lens with the cylindrical lens height of (a) 1 mm, (b) 1.28 mm, (c) 1.44 mm and (d) 1.6 mm.
Fig. 7. Comparisons of the geometry, the ray fans, and the divergent angles between the CLA-TIR lens (a), (c), (e) and the CLAS-TIR lens (b), (d), (f).
5. Experimental verification
The CLAS-TIR was made by CNC machining on a PMMA block medium, as shown in Fig. 8. The measurement was divided into two parts. One was for the total flux and the other was the luminous intensity distribution. The drive current was set 350 mA, and the pcW-LED was attached on a board to spread the heat. The steady-state flux was 155 lm, while the measured flux was 151 lm when the lens was attached on the pcW-LED.
The measurement of the luminous intensity was different. The pcW-LED with the lens was attached on a rotational stage and the exit surface was located at the rotational center. As shown in Fig. 9, a lux-meter at 4 m away from the LED module was used to measure the illuminance angle by angle, so that the luminous intensity can be calculated, as shown in Fig. 10, where the measured luminous intensity distribution was similar to the simulation, but the axial intensity was lower than that in the simulation. The intensity degradation was caused by machining error at the turning points of the lens.
Fig. 10. The rotation direction, measured intensity and normalized intensity distribution of the CLAS-TIR lens in the measurement along horizontal direction shown in (a), (b), (c) and vertical direction shown in (d), (e), (f).
To build up the complete marine signal light, we may use five pieces of the LED module. To reach 8 nautical miles (NM), the minimum luminous intensity should be larger than 445 cd. Figure 11 shows the simulation based on the design and the calculation based on the measurement. Owing to intensity degradation caused by machining error in the real sample, the injection current in the real sample should be higher than in the simulation to achieve similar intensity level. The luminous flux of each pcW-LED in the real sample needs to increase to 185.8 lm to build up a marine signal light to reach 8 nautical miles (NM) as in the design target. It means that the total power of the pcW-LED will be only 6W. Besides, by using the proposed CLAS-TIR lens, we compare the effects of the horizontal round-view illumination in the simulation when the piece of the LED module is added from 5 pcs to 6 pcs, as shown in Fig. 11(c). The angular spacing between the neighboring CLAS-TIR lens in the marine signal light using 5 pcs CLAS-TIR lenses and 6 pcs CLAS-TIR lenses are respectively 72° and 60°. According to the characteristic of the light pattern of the proposed CLAS-TIR lens, there is a large variation in the luminous intensity among different angles when 6 pcs LED module is used. Obviously, the light pattern is bounded by the number of the LED. Therefore, by considering the energy-saving effect, cost structure, heat disippation and the requirement of the minimum luminous intensity of IALA regulation, we adopt five pieces of the LED module in the proposed marine signal light.
Fig. 11. (a) in the design, the marine signal light contains five LED modules. (b) Comparison between the light pattern in the simulation and the experiment, where the luminous flux of each pcW-LED is 172.8 lm in the simulation, and 185.8 lm in the experiment. (c) Comparison between the light pattern using 5 pcs and 6 pcs CLAS-TIR lenses in the simulation. Obviously, 6 pcs LED will cause peak intensity and loss uniformity.
6. Summary
In this paper, we have presented a new design of an energy-efficient LED-based marine signal light. We started from analysis of the regulation, and fixed the key parameters, including vertical divergent angle of 8°-10°, reaching distance of 8 nautical miles (NM), and only five pcW-LEDs in the signal light. We first built up a precise light source model with NCC’s of 99% at different mid-field distances. Then we used a TIR lens to collimate the emitting light, and to have a divergent angle around 9°. In order to extend the horizontal divergent angle, a cylindrical lens array was added on the top surface of the TIR lens to form a CLA-TIR lens. We further removed the cylindrical lens on two sides with a special-design slanted shoulders to form a CLAS-TIR lens, which avoided internal total reflection, and increased the optical efficiency from 63% to 84.2%. In addition, the slanted shoulders could not only rescue the light from total internal reflection, but also reshape the light pattern to form a flat-top intensity distribution. The horizontal divergent angle was extended from 45° to 67°, which fitted the design of using five pieces of pcW-LED. The light patterns measured in the experiment coincided with the simulation of the design except the normal intensity dropped to around 80% owing to machining error. Even so, the performance of the designed marine signal light was verified with the five pcW-LEDs operated at around 6W, when the luminous flux of each pcW-LED was 185.8 lm.
Funding
Ministry of Science and Technology, Taiwan (MOST) (106-2221-E-008 -065 -MY3, MOST105-3113-E-008-008-CC2).
Acknowledgments
The author would like to thank the Breault Research Organization for providing the ASAP simulation program.
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